What is this hollow brown capsule attached to a twig?
An empty sawfly cocoon! Sawflies are wasp-like insects who get their name from the saw-like female ovipositor (egg-laying structure) they use to cut into plant tissue and lay eggs. Many sawflies lay eggs in the spring from which caterpillar-like larvae emerge (sawfly larvae have six or more pairs of prolegs, while caterpillars have up to five pairs).
Elm sawfly larva and adult
Many sawfly larvae spin a cocoon and overwinter as pupae. The cocoons can be in soil or attached to branches. The cocoons are important winter food for small mammals. Tracks & Signs of Insects and other Invertebrates notes, “It has been estimated that a single short-tailed shrew consumes twenty-three thousand pine sawfly cocoons in a year.”
In the spring, the new adult sawflies chew a circle at one end of their cocoon to create a flip-top lid (which often stays attached) and emerge as adults to continue the cycle.
If the exit hole is smaller, circular, and off-center near one end, it’s likely a sign the sawfly pupa was parasitized by an ichneumonid wasp.
So interesting! The shrew must be full!